This invention relates to a process for obtaining tissue-protective implants prepared from titanium or a titanium-base microalloy. More particularly, the present invention relates to a process for obtaining tissue-protective devices of bone surgery, especially cheek bone, mandible and common surgery, implants prepared from a medical-purity metal titanium and/or titanium-base microalloy, containing at least 98% of titanium, wherein a biocompatible (tissue-protective) coating is established by anodic oxidation on the metal surface after degreasing and chemical or electrochemical etching.
It is a known process to coat the metal implant with a hydroxyapatite (HA) layer by plasma-spray. This method of coating with HA types is described in detail by W. R. Lacefield ["Hydroxyapatite Coatings" in: Ann. N.Y. Acad. Sci. 523, 72-80 (1988)] as well as by K. A. Thomas et al. ["The Effect of Surface Macrotexture and Hydroxyapatite Coatings on the Mechanical Strength and Histological Profiles of Titanium Implant Materials" in: J. Biomed. Mater. Res. 21, 1395-1414 (1987)].
In case of layers applied by plasma-spray the bone formation becomes more rapid, the period of recovery becomes shorter and the connection of bone with the implant is more solid. These advantages are accompanied, however, by some difficulties: the composition of the HA layer applied depends on the technological procedure employed, the uniformity of the coating as well as of the size and shape are difficult to ensure, the absorption of the layer cannot be followed up and results of long-lasting experiments (10 to 20 year) are not available yet.
In general, the adhesion of the layers to the implants is not satisfactory and the free, polyvalent, e.g. Ti.sup.4+ ions are toxic. In case of absorption into the bone of the HA layer a connection develops between the metal implant and the bone. These disadvantageous properties are discussed in detail in an article by L. Claes and K. Hanselmann: "Neue Biowerkstoffe, degradierbare Materialen und bioaktive Oberflachen," 2. Symp. Materialforschung 1991, pp. 155-180, 26-29th August 1991, Dresden.
According to the German patent specification No. 1,943,801 the tissue-protective behaviour of implant devices is improved by a coloured thin coating showing the interference colours of an oxide, nitride, carbide or carbon nitride. This process is based on the empirical recognition that the danger of inflammation following the implantation is reduced by using a coating. This patent does not contain, however, any detailed description of the process or essential information about the material quality.
There are known processes utilizing anodic oxidation of titanium for non-medical purposes such as that described in the German patent specification No. 2,216,432, according to which porous titanium oxide is prepared by using a sulfuric acid/hydrochloric acid electrolyte and a formation voltage lower than 46 V.
The colouring of titanium is carried out by using a sulfuric acid solution of 12 to 30% concentration and a formation voltage of 14 to 20 V according to the published Japanese patent application No. 56/58990. For the same purpose, a mixture of acetamide with one or more acid(s) is suggested by the published Japanese patent application No. 56/168894. According to the published Japanese patent application No. 62/161993 the oxidation of titanium is terminated by the heat-treatment of the anodic layer. According to the published Japanese patent application No. 63/18099 a coloured oxide layer is developed by the anodic oxidation of heat-treated titanium whereas according to the Japanese patent application No. 2/194,195 the oxide layer is formed by anodic oxidation carried out with a mixture of phosphoric acid and oxalic acid.
The known processes using anodic oxidation or their combination with heat-treatment result in coatings that are either porous or their thickness is lower than 1000 .ANG., so they are unsuitable to medical purposes.